14 



a— BACTERIAL VARIATION AND TRANSMISSIBLE 



AUTOLYSIS ; THE RELATION OF BACTERIAL ENZYMES 



TO BACTERIAL STRUCTURE. 



By Arthur Eastwood, M.D. 



Introduction. 



Careful investigation of cultures, grown on ordinary or on 

 selective media, has shown that variations are of common occur- 

 rence in most bacterial species. The variants, whilst retaining 

 the general characters of their species, exhibit differences in 

 one or more of the following attributes* : — appearance of the 

 colonies, morphology of individual bacteria, motility, fermenta- 

 tive capacities, capacity for growth, antigenic properties, and 

 virulence. 



Of the principles which determine these changes very little 

 is known, but certain laboratory data seem to be clearly established. 

 In the first place, variants can be produced in cultures derived 

 from a single cell or from single colonies, a procedure which has 

 been repeated so often that there can be no doubt as to its 

 accuracy; thus one has no difficulty in answering the question, 

 Have you really produced a variant, or was the " variant " 

 present in small numbers, which were unrecognised, in the material 

 with which you started ? The answer is that variants undoubtedly 

 arise de novo and that this fact has been proved by properly 

 controlled experiments, which show that variants are definitely 

 attributable to some change associated with bacterial growth. 

 In the next place, though variants observed in the laboratory 

 are not all of equal importance and though some of them may be 

 merely artefacts, in the sense that they are attributable to causes 

 not likely to be operative in nature, the main fact remains that 

 many of these variants are of great interest, because they throw 

 light on the biological properties of bacteria and are thus related 

 to the problems of infection, resistance and immunity. 



To take the most obvious instance. A strain is split up 

 into two types, the " normal " and a variant ; though each grows 

 equally well in subculture, when tested on animals it may be 

 found that the former has retained its virulence whilst the latter 

 has lost it. This observation is clearly of interest in discussing 

 the factors on which virulence depends. Then one may go a 

 step further. Under the operation of certain influences in vitro, 

 a culture may be separated into two types, the " sensitive " 

 and the " resistant," which do not grow equally well, even in the 

 test-tube. The former is much less viable and the sensitive 



* I am assuming, provisionally and for the sake of simplicity, that the 

 attributes of the normal bacterium are clearly and accurately defined. 

 This, however, is not always the case ; it is sometimes difficult to say which 

 type is " normal " and which is " a variant." 



